1. Field of the Invention
This invention relates to a vacuum ultraviolet excited green phosphor material and a light-emitting device using the same.
2. Description of the Related Art
At the present time, fluorescent lamps containing mercury are in common use as illumination. This mercury vapor lamp generates ultraviolet radiation using mercury vapor discharge and then causes 3 phosphor materials, R (red: red wavelength region with peak emission wavelength of 600 to 615 nm), G (green: green wavelength region with peak emission wavelength of 535 to 570 nm) and B (blue: blue wavelength region with peak emission wavelength of 440 to 470 nm) (hereinafter abbreviated as R.G.B), to emit light to illuminate a white fluorescent lamp. However, these fluorescent lamps use in their inside mercury harmful to the environment and therefore the development of mercury-free fluorescent lamps, which use rare gases such as xenon, are being actively pursued. The mercury-free fluorescent lamps containing xenon gas emit vacuum ultraviolet light using xenon discharge and cause the phosphor materials of 3 colors, R. G. B, to emit light to yield white light.
On the other hand, in conventional display units using a CRT system, their weight and thickness are becoming problematic with the recent growing tendency toward large-sized display screens and there have been strong demands for light-weight and thin type display units. As a result, the development of flat flat-panel displays such as plasma display panels and liquid crystal displays are being actively pursued.
In plasma display panels, color display is obtained by generating plasma by electrical discharge in rare gas, producing vacuum ultraviolet light through the generated plasma, and causing phosphor materials of 3 pixel colors, R, G and B, to emit light by the vacuum ultraviolet excitation.
In liquid crystal displays, on the other hand, since they are not self luminous type of displays, reflection type displays, which use external light, and backlight type displays, which use back light on the back side of liquid crystal, are used. In large-sized liquid crystal displays, however, backlight type is commonly used. To provide color display, a backlight emits white light, which contains R, G and B light, from the back side of a liquid crystal device and a color filter separates the white light transmitting the liquid crystal device into R, G and B.
The phosphor materials used in the plasma display panels and backlights of liquid crystal displays are also required to have optical characteristics such as luminance, chromaticity and aging characteristic of the same.
Japanese Patent Laid-Open No. 2003-297291 discloses cerium, terbium-doped lanthanum phosphate (LaPO4: Tb, Ce) and terbium-doped magnesium cerium aluminate (CeMgAl11O19: Tb) phosphor materials as green phosphor materials whose emission peak wavelengths are at 505 to 535 nm.
Further, Japanese Patent Laid-Open No. 2003-96448 discloses aluminum borate green phosphor materials represented by the formula: Y1-a-bGdaTbbAl3(BO3)4 (0.3≦a≦0.55, 0.003≦b≦0.44) which have high luminance and are low in decrease of luminance due to their exposure to plasma.
Fluorescent lamps using xenon discharge, which have lately attracted considerable attention as mercury-free fluorescent lamps, are poor in energy conversion efficiency compared with fluorescent lamps using mercury discharge. To use them in household applications, they are required to have high luminance. If the luminance of xenon fluorescent lamps is increased to the same level as that of mercury fluorescent lamps and is put to practical use, power consumption can be reduced. Thus, it is indispensable to increase the luminance of phosphor materials.
Plasma displays and liquid crystal displays are alternatives to cathode ray tubes. To allow them to be used in homes, they are also required to have high luminance. However, in plasma display panels in current use, it is hard to say that they have luminance that fully meets such a requirement. In liquid crystal displays, since their luminance is obtained from their backlights, the requirement can be met by increasing the luminance of the backlights. However, from the viewpoint of power consumption, there have been demands for more efficient and thinner liquid crystal displays. Thus, it is indispensable to increase the luminance of fluorescent tubes which enable the easy formation of flat light-emitting devices.
Meanwhile, there have been strong demands for higher luminance and higher color purity of the green phosphor materials shown in the description of prior art.
Demands for increase in luminance and improvement in color purity are particularly noticeable in plasma display panels. This may have to do with the fact that the system of TV broadcasting is moving toward the Hi-Vision digital production system from the conventional NTSC system. The Hi-Vision digital production system requires 1920×1080 pixels Full Spec; however, the current 37″ to 42″ plasma display panels have only 1024×768 pixels. And if plasma display panels are produced while maintaining the current pixel sizes, the resultant panels are 55″ to 63″ ones. For around 40″ plasma display panels, which are of size commonly used in homes, to comply with the standard of Hi-Vision full spec, it is necessary to decrease the pixel size to ⅔ or less of the current one. If the pixel size is decreased to ⅔ of the current size, the luminance of the phosphor materials is required to be 1.5 times or more as much as that of the current luminance. Thus, it is indispensable to increase the luminance of the phosphor materials.
There also exists a problem of color purity with conventional phosphor materials. For example, when the green phosphor materials have blue- or red-emission, color impurity may occur.
This invention is intended to provide phosphor materials that overcome the above described problems.